This invention relates to variable reluctance motors and, more particularly, to single-phase variable reluctance motors in which a motor pole is shifted to improve the torque profile of the motor.
In U.S. Pat. No. 5,122,697, which is assigned to the same assignee as the present application and which is incorporated herein by reference, there is disclosed a hybrid single-phase variable reluctance motor. A particular problem which the motor described therein is designed to solve is improvement in starting of the motor. This is because these types motors have a minimum reluctance position from which, if the rotor stops in that position, it is difficult to generate torque to start the motor. While various embodiments of the invention are described, in general, the motor is designed to have the rotor come to rest at a preferred position from which it is easy to start the motor.
While the motor of this invention works well for its intended purpose, some problems still persist. One, for example, is that the rotor may still have a stable detent or minimum reluctance position from which it is difficult to start the motor if the rotor "parks" there. While this problem is no longer as severe as in prior art single-phase variable reluctance motors which the motor of the 5,122,697 patent is designed to replace, it nevertheless can still be occur. One contributor to this problem is the type of bearings used in the motor. Typically these are either ball bearings or sleeve bearings. Each type bearing has a frictional load band. The actual amount of friction is a function of motor speed; but in general, at normal operating speeds, self aligning sleeve bearings are subject to 2-3 times the amount of friction as roller bearings. There is a critical region where the rotor must not stop if the motor is to start again. This region is where both the Hall effect device is not activated and where negative torque by the coil excited poles would result. The magnet torque available in this critical region must be sufficient to overcome the load friction due to the bearings or the rotor will stop in this area. The motor will not start if this should occur.
A second problem deals with the noise generated by motor "ovalizing". Motors are made such that the stator and rotor assemblies are installed within a shell or frame. In the typical two-pole motor construction of single phase variable reluctance motors, forces are produced as the rotor and stator poles come into alignment which produce an inward or pulling force on the lamination back iron at the location of the poles, and an outward or pushing force on the shell and lamination back iron at the point 90 electrical degrees from the poles. These forces cause the shell, which is generally circular in cross-section to distort into an oval shape. The metal flexure resulting from the distortion produces noise. In tests, this noise level has measured as high as 45 dba. This is an undesirable level.